The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio, at which ratio a three-way catalyst acts most effectively.
In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O.sub.2 sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio for producing a control signal. The control signal is applied to a proportion and integration circuit (PI circuit), the output of which is changed to pulse form. The pulses operate an electromagnetic valve so as to control the amount of bleed air in a carburetor for controlling the air-fuel ratio of the mixture. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio at which a three-way catalyst in the exhaust system acts most effectively. In such an air-fuel ratio control system, when the temperature of a sensor body of the O.sub.2 sensor is lower than a predetermined temperature, the O.sub.2 sensor does not act as a sensor. Accordingly, until the O.sub.2 sensor is activated by the temperature of the exhaust gases, the duty ratio of the pulses is fixed to a predetermined value which is selected to control the air-fuel ratio under conditions of combustion at low temperature (25.degree. C.) of the engine. When the operation of the engine is stopped, the temperature of the sensor decreases quickly compared with the temperature of the engine including the carburetor. When the engine is restarted while the engine temperature is at high temperature, the carburetor supplies a rich air-fuel mixture to the engine because of the high temperature of the body of the carburetor. On the other hand, if the O.sub.2 sensor is not activated, the duty ratio is fixed, which means that the feedback control system does not operate. As a result, the air-fuel mixture is extremely enriched. The rich mixture will cause difficulty in starting the engine, high exhaust emissions, poor fuel economy, and other problems.